Illumination systems that contain blackbody light sources such as arc lamp sources or incandescent sources are usually designed so that no light is reflected or recycled back to the source. Blackbody light sources are excellent light absorbers and poor light reflectors. Any emitted light that does get back to the source is absorbed and lost, lowering the overall efficiency of the illumination system.
Certain types of light sources, such as some fluorescent light sources and some light emitting diodes (LEDs), can reflect light as well as emit light. Reflecting light sources can be used in illumination systems that recycle light back to the source. Recycled light that is returned to the source and that is reflected by the source can increase the effective brightness of the source. In addition, light sources that can reflect light instead of absorbing light can reduce absorption losses and increase the overall output efficiency of such illumination systems.
U.S. patent application Ser. No. 10/445,136, U.S. patent application Ser. No. 10/814,043 and to U.S. patent application Ser. No. 10/814,044 describe light recycling systems that include light recycling cavities or envelopes that enclose one or more light reflecting LEDs. The light reflecting cavities or envelopes reflect and recycle a portion of the light emitted by the LEDs back to the LEDs. The light recycling cavity or envelope has an output aperture with an area that is smaller than the total emitting area of the enclosed LEDs. In such cases, it is possible for the light exiting the cavity or envelope to be brighter than an equivalent LED measured in the absence of recycling. The three aforementioned applications disclose illumination systems that incorporate light recycling cavities or envelopes, but do not disclose illumination systems that lack light recycling cavities or envelopes.
U.S. patent application Publication No. 20020093284 describes a light-emitting display device with a reflective element behind an organic light-emitting layer. Cholesteric liquid crystal polarization separators, a quarter-wave plate and a polarizer are formed on the organic light-emitting layer. The light-emitting device is a display device. Any ambient light from outside the device, which is incident on the device, is absorbed by the polarizer to prevent reflection of the ambient light. The polarization separators reflect circularly polarized light, which is converted to linearly polarized light; a portion of the linearly polarized light is absorbed by the polarizer.
It would be desirable to design illumination systems that utilize light emitting diode light sources and light recycling but that do not require a light recycling cavity or envelope. It would be desirable to design illumination systems that maximize emitted light and maximize the reflection of incident light. Such systems can have increased output brightness and efficiency compared to systems that do not recycle light.